In the heat treatment of pumpable liquid food products, the product is heated to a predetermined temperature. The temperature depends on the type of process that is intended, such as, for example, pasteurisation or sterilisation. When the product has reached the desired temperature, it is to be kept at this temperature for a given, predetermined interval of time. By combining temperature with stay time for different products, there will be obtained products with different shelf lives, such as sterile products that can be delivered at room temperature and such products as are to be delivered in an unbroken refrigeration chain.
The heat treatment may be put into effect indirectly or directly. In indirect heat treatment, the heating takes place in some form of heat exchanger, for example a plate heat exchanger or a tube heat exchanger. In the direct methods, steam is supplied direct to the product. Examples of direct methods are injection heating and infusion heating.
In order to be able to maintain the temperature which the product obtains as a result of the heat treatment during a given predetermined interval of time, use is made of a holding tube which is placed in immediate association with the heat treatment equipment. The holding tube is normally designed as a pipe loop, or alternatively a straight pipe length, or as a combination of both. The length of the pipe or the pipe loop corresponds to the time which for a given product at a given calculated capacity stays in the holding tube. It is important that the stay time be exact, since too short a stay time does not give the desired treatment of the product, with the result that a product is obtained which does not have the intended shelf life and which may rapidly become a downright health hazard. Excessively long stay times can destroy the product which, as a result of excessive thermal shock, suffers from changes in flavour, aroma or nutrient content.
Normally, temperature indicators are provided in holding tubes, the indicators being placed ahead of and after the holding tube, where the temperature indicator placed ahead of the holding tube is included in the control of the regulator loop for the heat treatment equipment. The temperature indicator placed after the holding tube is used to monitor that the product was at the correct temperature in the holding tube. On the other hand, at present there are few reliable methods of checking that the stay time is that intended.
The traditional methods that consist of the conductivity method and the dye method cannot be employed during production. The product must then be replaced by water and salt or a dye, respectively, is added to the water. Thereafter, the time it takes for the saline solution or the dyed water, respectively, to pass through the holding tube is measured. The saline water is registered by conductivity measurements and the dyed water is registered by means of observation. Both of these methods suffer from numerous drawbacks and the error sources are obvious.
Patent Specification WO 01/98738 discloses a method of monitoring the stay time in a restricted pipe length by inducing a temperature change in the product. This temperature change is registered by one temperature gauge ahead of the holding tube and one temperature gauge after the holding tube. The time which elapses between these registrations constitutes the stay time. The drawbacks inherent in this method are that it is necessary to induce a disruption which is sufficiently great for it to be able to be measured in a single measurement. There is always a danger in disrupting a process and a wealth of knowledge is required to be able to carry this out reliably.